Catheter based angiography is a medical imaging modality offering planar viewing of contrast enhanced blood vessels and other structures within the human or animal body. For many applications it is the standard technique used for vascular imaging and as a result there is a large scale standardization of the standard viewing planes for particular anatomical structures. The standardisation of viewing planes is however a guide and the use of such standard viewing planes may not achieve the ideal representation of a three dimensional anatomical structure using a two dimensional plane.
Catheter based angiography can be used to image a segment of an anatomical structure for subsequent use in assessment of any abnormalities that may be present, for example determining whether a stenosis is present in a segment of a coronary artery. The technique can also be used as part of a medical treatment being performed on a patient, for example it can be used to image a coronary artery whilst a stent is being inserted into the coronary artery.
Standard catheter based angiography techniques use a C-arm X-ray imaging apparatus, in which an X-ray source and detector are positioned at opposite sides of a patient's body. A catheter is inserted to a region of interest and contrast agent is injected. The X-ray source and detector can be positioned at any orientation with respect to the patient's body to provide a two-dimensional planar image at any desired orientation. Blood vessels in which the contrast agent is present can be seen clearly on the resulting image.
Catheter based angiography is widely used for viewing of coronary artery structures. In particular, catheter based angiography is widely used to detect the presence of stenoses (abnormal narrowings) in coronary artery structures. In order to detect the presence of a stenosis in a particular coronary artery structure planar images at various orientations, including at least two orthogonal orientations, are obtained and the resulting planar images are viewed by a radiologist or cardiologist who is trained to detect the presence of stenoses or other abnormalities from the images.
The coronary arteries make up a complex tree-like structure, which can be segmented into different longitudinal segments according to standard models, for example the AHA (American Heart Association) 15 segment model or the modified AHA 17 segment model. Viewing planes for the viewing of different segments of a normal coronary artery structure using catheter based angiography have become standardised and there are standard viewing planes for each of the segments.
For each segment, catheter based angiography measurements are performed for the standard set of orientations of the X-ray source and detector for that segment to obtain images for each of the standardised viewing planes. Typically for each vessel, measurements at 6 or 7 different orientations may be obtained. A radiologist or cardiologist is trained to detect the presence of stenoses or other abnormalities based upon these standard viewing planes.
Human anatomy varies from patient to patient and the standard viewing planes for coronary catheter-based angiography represent the best viewing planes for the average patient, and do not take into account individual variation between patients. For some patients, the cardiac artery anatomy will differ significantly from the average, and a radiologist or cardiologist may instruct further catheter based angiography measurements to be performed at different orientations, after viewing images obtained for the standard viewing planes. Significant amounts of time can be wasted in adjusting views to obtain an optimum viewing plane for each patient.
Given the number of different segments of the coronary artery structure, the number of separate catheter based angiography measurements required for each segment, and the possibility of having to repeat measurements at different orientations for some patients, catheter based angiography for the study of coronary artery structures can require a large number of separate measurements, which can be time-consuming and expensive. Furthermore, as catheter based angiography is an invasive procedure it can cause complications in some patients. In addition, catheter based angiography can be used only to obtain information on the lumen of the coronary artery or other blood vessel, it cannot generally be used to detect the thickening of the vessel wall.
Modern three-dimensional imaging techniques including computerised tomography (CT), magnetic resonance imaging (MRI) and volumetric (cone-beam) angiography, have the ability to produce volumetric representations of anatomy allowing users to examine acquired data retrospectively or under live screening from any plane and apply image processing techniques to achieve accurate viewing of individual structures. CT and MRI measurements are generally quicker and cheaper to perform than traditional catheter based angiography measurements, and can also be used to determine the thickness and other properties of vessel walls as well as the path and thickness of the vessels themselves.
Such three-dimensional techniques produce large three-dimensional volume data sets comprising a three-dimensional array of voxels each representing a property of a corresponding measurement volume. In the case of CT data sets, each voxel usually represents the attenuation of X-ray radiation by a respective, corresponding measurement volume.
Many techniques have been developed for selecting, processing and visualising data obtained used three-dimensional imaging techniques. For example, multi-planar reconstruction (MPR) techniques can be used to select and visualise two dimensional planes from the three dimensional data set. A plane is selected within the three dimensional volume and data from that plane only are displayed. Slab (or thickened) MPR is a variant of the technique in which the MPR plane has a selected thickness of greater than one voxel and data only from the thickened plane are displayed. Various rendering techniques determining how the MPR data are rendered on a display are also know, for example maximum intensity projection (MIP) or direct volume rendering (DVR).
In the case of coronary artery imaging using three-dimensional imaging techniques, there are currently two main recognised methods of investigating the coronary arteries in cross sectional imaging, based on three dimensional data sets obtained from coronary computed tomography angiogram (CCTA) measurements or cardiac magnetic resonance imaging (CMR) measurements.
The two methods take different approaches to the difficulties of representing portions of the curved, complex coronary artery structure in two dimensional images.
In the first method (also referred to as the traditional approach) planar two-dimensional images are obtained from slab (thickened) MPR of a selected segment of the coronary artery structure, and are used to examine the segment of the coronary artery along its length. Usually, the segment of coronary artery to be viewed, and the orientation and thickness of the MPR plane, are selected manually by an operator using a suite of imaging and rendering tools.
Such planar MPR techniques are generally trusted by users but each planar MPR image can only provide a partial view of a curved structure such as a coronary artery structure. It is also time consuming for an operator to select appropriate plane orientations and thickness to adequately view a segment of coronary artery structure, and the manual nature of the procedure can produce variations between images.
The second method is a curved MPR method, which uses curved plane reconstruction. The centreline path of a vessel, in this case a segment of coronary artery, is calculated along its entire length and then viewed (rendered) as a single extruded plane with the associated perpendicular axial cross sections of the curve at any selected point along its length being displayed.
Curved MPR can provide a good overview of the entire vessel in a single planar view, but curved MPR images are generally not as trusted by users as planar images are. Furthermore, it is still necessary for the user to select slab thicknesses and other parameters when using curved MPR. The selection of such parameters and the selection and viewing of cross-sectional images along at a series of manually selected points along the vessel are time consuming.
Coronary vessels are inherently curved structures, which change planar orientation significantly along their course and provide significant challenges to users in obtaining and interpreting two dimensional images of the structures from three dimensional data sets using CT, MR or cone-beam catheter based angiography methods. The time taken to obtain and interpret two dimensional images using either the traditional or modern methods described above can be significantly greater than the time needed to interpret images obtained using traditional catheter-based angiography techniques. Furthermore, curved MPR techniques are at present not trusted by some practitioners.